Holography is technology for recording an interference pattern on a light receiving surface of a photographic plate by receiving on this plate both an object light generated by illuminating a model object with an illuminating light and a reference light coming from another pathway, in which the illuminating light and the reference light are mutually coherent and are prepared by splitting a laser light. A hologram is a photographic plate after development. A three-dimensional object image (virtual image) can be reconstructed and seen in a position where there was the object by irradiating the hologram with a light for reconstruction. Computer holography is technology for calculating data of an interference pattern on an arbitrary plane defined as a hologram plane, on which an object is expressed by data inside a computer, and physical simulations of reflection, diffraction, and interference of light are made within the computer. A hologram is realized using a certain display device based on the calculation result. Since an object is defined with data on a computer, no model object is necessary and optical compensation is possible. Moreover, object images can be reconstructed continuously by realizing a hologram by displaying the interference pattern recorded on a memory on a reflective type LCD etc. one after another and by irradiating the hologram with a reference light.
Digital holography is technology for generating a hologram by sensing and recording an interference pattern with a CCD image sensor, a CMOS image sensor, etc. electronically instead of with a photographic plate, and processing a light intensity distribution of the recorded interference pattern numerically to compute an object light wave. If digital holography is used, the wave front data of an object light can be obtained in a form of a two-dimensional complex amplitude distribution on an image sensor surface. Object images in various viewpoints and positions are obtained based on wave front data like above-mentioned computer holography. By the way, in a three-dimensional image pickup by holography, generally, a reference light and an object light themselves and also optically mixed interference fringes, which are made by mutual mixture of object lights scattered on a object surface, and mutual mixture of reference lights generated by scattering on a light pathway, and so on, are recorded in addition to optically modulated interference fringes made by modulating the reference light with the object light. Among these, the requisites for image reconstruction are the optically modulated interference fringes, and other light components have an adverse effect on image reconstruction and reduce image quality of a reconstructed image. In order to acquire only the optically modulated interference fringes and to record them as object light wave front data, it is necessary to acquire above-mentioned complex amplitude distribution. The hologram using the conventional photographic plate etc. is of only real part data, because it fixes and records phase information of object light with a method so called interference instead of recording momentary phase of a light wave (object light wave front information). Complex amplitude consists of plural data called amplitude and phase, or real part and imaginary part. Therefore, in order to obtain complex amplitude, a plurality of hologram data are required. Phase shift digital holography is known as technology for acquiring only the optically modulated interference fringes (for example, refer to patent document 1).
In the phase shift digital holography, phase state of a reference light to an object light is shifted in three steps or four steps, and a plurality of mutually different hologram data are acquired. There are several methods to shift the phase of the reference light such as a method of inserting a thin glass plate in a propagation path of the reference light and a method of moving a position of a mirror which reflects the reference light using a piezoelectric element. For example, using the piezoelectric element, three kinds of hologram data are acquired in the state where the phase of the reference light is shifted every π/2, and complex amplitude of the object light is obtained from simultaneous equations between each picture element data of three holograms. However, in an image recording using this method, since an amount of shift of control parameter for a phase shift generally has wavelength dependency, there is a fault of difficulty in color hologram acquisition. A phase shift method using a spatial light modulation element is proposed as a way to shift simultaneously only the same value for phases of 3-color reference lights of red, blue, and green by annihilating the wavelength dependency (for example, refer to patent document 2).
However, although the phase shift digital holography using the spatial light modulation element is excellent as image pick-up technology of a still three-dimensional image, it is necessary to shift the phase states of reference light and to record a plurality of holograms one by one to obtain single complex amplitude hologram. That is, this recording is in principle done in time transition, unless simultaneous recording is done for several holograms of different phase states. Accordingly, there is a limit to applying this technology to pick-up of three-dimensional image of a photographic subject which changes temporally or a photographic subject which moves. Then, there is a method for acquiring data of a plurality of holograms simultaneously by changing phase states of reference light for every picture element of a light receiving surface. For example, a phase distribution in a cross section of the reference light is given by inserting, in the propagation path of the reference light, a phase shift array device which has elements arranged in an array so that adjacent one is of different phase mutually by π/2, and information of an interference pattern, in which phase of the reference light is changed in four steps; 0, π/2, π, and 3π/2, is recorded in a single hologram. In this case, every one fourth of the picture element data in the single hologram forms four kinds of holograms, respectively (for example, refer to patent document 3).
The method of using the phase shift array device needs to coincide strictly each picture element position of an optical deflection array device, the phase shift array device, and a photo detecting device, and therefore high alignment accuracy is required for the devices. Moreover, there is an issue such as error correction when a picture element position shifts due to disturbance. Moreover, it seems to be difficult to make such a phase shift array device with a high degree of accuracy at low-cost. Furthermore, since the phase shift of light has wavelength dependency, it is thought that colorization of a recorded image is difficult. Then, a method for realizing high-speed acquisition of a phase shift hologram by an easy composition is proposed, which changes phase states of the reference light on a light receiving surface for every picture element only using a geometric arranging relation of the parallel reference light and the light receiving surface (for example, refer to patent document 4).
The method shown in the above-mentioned patent document 4 enables single shot record of a color three-dimensional image by inclined irradiation of the parallel reference light, and post processing of hologram data. This method, by irradiating a photo detector such as CCD with the inclined parallel reference light, makes the phase of reference light distribute periodically on the photo detector surface, and records an object light wave front by a single shot as an off-axis hologram. Then, three or four interference fringe holograms each having different phase state of the reference light are derived by data processing such as a spatial sampling of recorded hologram data and a data interpolation. By using the derived plurality of interference fringe holograms, a complex amplitude in-line hologram which records only object light wave front after elimination of a noise component is generated. Since this complex amplitude in-line hologram is obtained based on a single hologram of a single shot record without time transition in this method, real-time image pick-up of three-dimensional image in motion is in principle attainable by improvement of processing speed and use of pulsed laser.